The definition of photocatalysis is basically the acceleration of a
photoreaction by the presence of a catalyst. A more in depth approach would
include that the catalyst may accelerate the photoreaction by interaction
with the substrate in its ground or excited state and/or with a primary
photoproduct, depending upon the mechanism of the photoreaction [1].

Catalysis by definition, implicates a catalytic entity that participates
and accelerates the chemical transformation of a substrate, itself remaining
unaltered at the end of each catalytic cycle [1]. In photocatalysis, no
energy is stored; there is merely an acceleration of a slow event by a
photon- assisted process.

It would probably be a good time to introduce a few of the many ways
in which photocatalysis works in basic easy to understand terms. In figure
one; M is the metal containing catalyst or catalyst precursor, O is the
organic reactant, P is the product, C’ is the photoassistor also pseudocatalyst,
R is the primary photoproduct, and hv is the irradiation via ultraviolet
or visible light. The simplest most basic equation is that the irradiated
subject is changed to an excited state thus increasing the ease of bond
making and braking which ultimately renders the organic reactant to a desired
product. As previously mentioned figure one includes schemes 5-9. Scheme
5 illustrates the process, commonly termed photosensitization, in which
the interaction between electronically excited M and ground-state substrate
activates the latter and regenerates M. In scheme 6 the reaction of M*
(the exited M) produces a ground state species, C’, which assists the transformation
of substrate to product and then reverts to M. Scheme 7 describes the case
in which M catalyzes the reaction of an electronically excited organic
substrate via formation of an excited complex, M-O*. Scheme 8 involves
a metal-catalyzed reaction of a primary photoproduct, R. Lastly, Scheme
9 illustrates a transformation that results from irradiation of a ground
state M-O complex [1].

Machines or specially designed equipment are needed in this field because
inconveniences and detrimental factors in direct solar photolysis are the
lack of sunlight absorption by the substrates, attenuation of the sunlight,
and the relatively shallow penetration depth of sunlight in natural aquatic
bodies.

There are many types of catalyst, some act on very few substrates while
some act on many substrates. The best way to cleanse a wastewater would
be to use a photocatalysis process that can be effective on a multitude
of contaminants or in other words a heterogeneous environment of contaminants.
Metal oxides work well in this case. It is true that many oxides work well,
WO3, and ZnO but in scientific studies it has been proven that
TiO2 has an advantage over the others.

The reasons that TiO2 does so well and is desired as an agent
in remediation of wastewater is based on several factors. 1. The process
occurs under ambient conditions. 2. The formation of photocyclized intermediate
products, unlike direct photolysis techniques, is avoided. 3. Oxidation
of the substrates to CO2 is complete. 4. The photocatalyst is
inexpensive and has a high turnover. 5. TiO2 can be supported
on suitable reactor substrates. 6. The process offers great potential as
an industrial technology to detoxify wastewaters [1].

Current Uses of TiO2 Photocatalysis

Researchers have used photocatalytic oxidation (PCO) to break down and
destroy many types of organic pollutants. It has been used to purify drinking
water, destroy bacteria and viruses, remove metals from waste streams,
and breakdown organics into simpler components of water and CO2.

After photocatalysis was realized to be a great oxidation mechanism,
researchers began testing it on many different compounds, and in many different
processes. To date, this technology has been used to detoxify drinking
water, decontaminate industrial wastewater, and purify air streams.

Photocatalytic Treatment of Water

Some of the first experiments showed that chlorinated aliphatic hydrocarbons
were dechlorinated and mineralized [2]. This means that the compounds were
broken down into water and CO2. Before long researchers realized
that this advanced oxidation technique could be used on many compounds,
including some aromatics that are resistant to normal oxidation reactions
[2]. According to Purifics, an industrial water treatment company specializing
in photocatalysis, many different chemicals have proven to be detoxified
or removed from water [3]. These chemicals include:

Organic FamiliesToxic Compounds

alkanes PCB’s

alkenes PAH’s

alkynes dioxins

ethers furans

aldehydes pesticides

ketones herbicides

alcohols phenols

amine compounds cyanide

amide compounds

esters

Treatment of water can be accomplished by adding a powdered form of TiO2
to the water, or it can be immobilized on a substrate. If TiO2
is in solution then some sort of recovery system is necessary in order
to reuse the catalyst.

Photocatalysis has not only been proven to remove pollutants from water,
but also nuisance color, taste and odor compounds [4]. Tests have also
proven TiO2 to effectively remove bacteria, and viruses from
water supplies. A study by Ireland et. al. showed that TiO2
oxidation effectively removed Escheria coli (E. coli) from drinking water
[5].

Photocatalytic Treatment of Air

Treatment of polluted air streams is often more efficient that treating
liquid waste streams. Gas phase kinetics allow reactions to occur much
faster than in the liquid phase. This fact has lead some people to utilize
air stripping of pollutants from liquid phase for treatment in the gas
phase. In the process of treating air streams, TiO2 must be
suspended on some sort of surface to allow the gas to pass over it and
react. This is usually some sort of matrix with a high surface area, which
the UV light is shown upon.

An air treatment system for ethylene removal has been developed at University
of Wisconsin-Madison [6]. This system will be placed in produce sections
of grocery stores to remove the naturally occurring ethylene that causes
fruits and vegetables to spoil. The UV light has also shown to reduce bacteria,
molds and odors [6].

Application of TiO2 Photocatalysis

A common application for TiO2 photocatalysis is the mineralization
of trichlorotmehtane (CHCl3). Trichloromethane is a suspected
carcinogenic chloroform produced from dissolved organic matter during conventional
water chlorination procedures[2]. This purification process is shown to
be very effective in an experiment performed by David F. Ollis, a chemical
engineer at North Carolina University. According to his results published
in Environmental Science and Technology, "The simultaneous presence of
illumination and TiO2 produced the chloride ion and caused the
disappearance of chloroform" [2]. The basic general equation for chloroform
breakdown is given below:

H2O + CHCl3 + (1/2)O2 => CO2
+ 3HCl

The oxygen needed for the experiment is aerated throughout the contaminated
water. The statement given states that a chloride ion is produced. Figure
II shows that the ions combine with hydrogen to form a more desirable compound
HCl [2]. Ollis’ experiment shows great promise and provides ample information
to show its success. Figure II also shows the differences in using only
one of the procedures at a time compared to using both procedures at the
same time. The results are staggering showing a combined effort associated
with a drastic drop in tricholormethane. Overall the experiment is well
documented and explained. One problem addressed when using the insoluble
catalyst TiO2 is the need to recover and reuse this material.
After much research, we would suggest experimenting with immobilizing the
catalyst perhaps placing TiO2 in glass. Immobilizing the catalyst
can cause a wide range of problems from lower efficiency to difficulties
in mass transport, but that is beyond the scope of the paper.